This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. We are working to understand the relationships between African trypanosomes and their hosts. Our approach is to pursue a molecular and cellular understanding of the "Flagellar Pocket" (FP), a specialization of the trypanosome cell surface that surrounds the place where the flagellum protrudes from the parasite's surface. This is the site of all trypanosome endo- and exocytosis, so it plays a role in this parasite's strategies for avoiding attack by the host's acquired immune response. We are using electron tomography to pursue structural studies at about 6 nm resolution in 3-D. Our work is organized around the recent finding that a component of human LDL, ApoL1, is cytotoxic to some African trypanosomes but not others, the difference depending on a protein that is specific to their endocytic systems. Preliminary results suggest that this cytotoxic factors enters trypanosomes at the FP but somehow lead to cell lysis and death. We anticipate that this way of killing trypanosomes could be exploited for medical purposes. An understanding of mechanisms for both species-specific and broad-spectrum trypanosome killing by serum factors should provide insight into host-parasite biology and will assist in assessing novel routes to preventative treatments. We have therefore used electron tomography to examine the FP of T. brucei and found that FP-associated membrane components include clathrin-coated vesicles, tubulins, and a wide variety of membrane-bounded compartments. Data for 11 dual-axis tomograms have been collected to reconstruct these compartments in 3D. We are now working to develop labeling techniques that will characterize all the endocytic pathways, including those that internalize cytotoxic factors from mammalian serum.